Versatile microsystems for DNA amplification with polymerase chain reaction (PCR) and DNA analysis with capillary electrophoresis (CE) have become increasingly popular since such devices were first described (Northrup et al., “DNA amplification with a microfabricated reaction chamber,” Proceedings of the 7th International Conference on Solid-State Sensors and Actuators, Yokohama, Japan, pp. 923-926, 1993). These instruments are typically manufactured as sandwich chips or wafers from silicon, glass, or plastic substrates, by using scalable microfabrication techniques originated in the semiconductor industry. Microfabricated devices permit limitations of conventional PCR (long assay times, large and/or expensive volumes of reaction components, etc.) to be overcome. Examples of recent designs include fully integrated PCR-CE microfluidic devices (See, e.g., Lagally et al., Sensors and Actuators B, 63:138-146, 2000; Lagally et al., Analytical Chemistry, 73:565-570, 2001; Lagally et al., Lab on a Chip, 1:102-107, 2001; and Lagally et al., “Monolithic integrated PCR reactor-CE system for DNA amplification and analysis to the single molecule limit,” 2nd Annual IEEE-EMBS Special Topic Conference on Microtechnologies in Medicine and Biology, Madison, Wis., 2002), and 384-lane CE arrays (Emrich et al., Analytical Chemistry, 74:5076-5083, 2002).
Further progress in design, development and use of on-chip DNA analysis technology is hampered, however, by difficulties derived from inherent properties of microfluidics and from the handling of microfluidic devices. Problems occur for instance when connecting to macrodevices, loading very small volumes, and mixing several components. When using the devices of the prior art, these steps frequently result in air encapsulation or forming of air bubbles, which in turn clogs the microfluidic channels leading to loading disturbances and contamination of samples and reagents or reagent mixtures. Prior to the development of the present invention, this problem was addressed by employment of various additional precautionary devices (e.g., vacuums, pumps, microfluidic valves and vents) that have made the technology less versatile and more expensive. Other microfluidic systems of the prior art have addressed the air bubble problem by using capillaries containing filaments that can be loaded by micropipette, microelectrode, etc. (Brown and Flaming, Advanced Micropipette Techniques for Cell Physiology, Sutter Co., 2001). Although the use of the capillary with filament structure allows one to fill the capillary with more than one liquid component without the introduction of air bubbles, this design results in the sequential loading of the capillary, and difficulties in mixing the different liquid components. Thus what is needed in the art, are less expensive and cumbersome microfluidic tools that permit bubble-less liquid loading, as well as complete mixing of different liquids.